Conventionally, a magnesium alloy is known as a nonferrous alloy used as a material of automobile parts or aircraft parts (for example, refer to International Publication No. WO 2013/180122 and Japanese Patent Application Publication No. 2012-057227). Characteristics of a magnesium alloy include the lightness of specific gravity and the superiority in specific strength, specific rigidity and heat conductivity, as compared with another main alloy, such as an aluminum alloy or an iron alloy.
Furthermore, a magnesium-lithium (Mg—Li) alloy in which lithium is added, as a metal to be added, to a magnesium is known (for example, refer to International Publication No. WO 2009/113601 and Kazuo MATSUZAWA, Toshio KOSHIHARA and Yo KOJIMA, “Age-hardening and mechanical properties of Mg—Li—Al alloys”, Journal of The Japan Institute of Light Metals, 1989, Vol. 39, No. 1, p. 45-51). The Mg—Li alloy has the advantages that the density is low and workability is satisfactory as compared with a magnesium alloy. Mg-14 wt % Li-1 wt % Al alloy, called LA141 standardized by the ASTM International which is a standardization organization, is known as a standardized Mg—Li alloy.
Moreover, although the crystal structure of magnesium is the α phase which has the hexagonal-close-packed (hcp) structure, it is known that the crystal structure of magnesium becomes a mixed phase of the 0 phase, whose crystal structure is the body-centered cubic (bcc) structure, with the α phase when lithium is contained 6 wt % to 10.5 wt %. Furthermore, it is also known that the crystal structure of magnesium becomes the β-single-phase when lithium is contained not less than 10.5 wt %. That is, the crystal structure of an Mg—Li alloy is changeable by the content of lithium being changed.
According to International Publication No. WO 2009/113601, it is reported that the β single-phase is obtained when a composition of an Mg—Li alloy is not less than 10.5 wt % and not more than 16.0 wt %, and preferably not less than 13.0 wt % and not more than 15.0 wt % of lithium, and an alloy which is light and excellent in a cold workability can be made. Also, it is reported that when the aluminum is contained not less than 0.50 wt % and not more than 1.50 wt %, mechanical strength can be improved. Furthermore, it is reported that when an Mg—Li alloy ingot is subjected to a cold plastic working, and subsequently, to an annealing at 170° C. to 250° C., corrosion resistance and cold workability can be improved. Note that, annealing is a heat treatment aimed to remove strain arising due to work hardening and to improve ductility.
Meanwhile, in Kazuo MATSUZAWA et al., an Mg—Li alloy consisting of the α-single-phase, an Mg—Li alloy mixed of the α phase and the β phase, and an Mg—Li alloy consisting of the β-single-phase have been made respectively, and the result of examinations of rolling workability, aging hardening property and mechanical property has been reported. Specifically, three kinds of Mg—Li alloy ingots having the a-single-phase, a mixed-phase of the α phase and the β phase, and the β-single-phase were rolled at room temperature with process annealing, and subsequently, solution treatment and aging treatments were performed. As a result, it has been reported that the cold rolling workability of the β-single-phase of Mg—Li alloy has been improved the most. In the examinations, the solution treatment after rolling has been performed at 390° C., and the aging treatments has been performed at 60° C., 100° C. and 150° C.
Note that, solution treatment is a heat treatment which progresses solid solution of alloy contents by heating an alloy and quenches the alloy to avoid generating precipitates. Meanwhile, aging treatment is a treatment which changes material characteristic of metal by time passage. The aging advancing at a normal temperature is called natural aging while the aging advancing at a higher temperature than a normal temperature is called artificial aging or tempering aging.
On the other hand, in recent years, the composite material attracts attention as a material alternative to a metal material. This is because the composite material is lighter than a standard metal, such as an aluminum alloy and also, the strength of the composite material is higher than that of the standard metal. Consequently, the use rate of composite material tends to increase, in particular in aircraft parts requiring both of weight-saving and keeping strength.
An object of the present invention is to develop a material having better property, as a material for an automobile part, an aircraft part or the like.